Testing the Intermediate Disturbance Hypothesis on Anthropogenic Pressure in Central Alabama

Author

• Gavin Terrell, Jacksonville State UniversityFollow

Date of Award

Spring 2026

Document Type

Thesis

Degree Name

Master of Science (MS) in Biology

Department

Biology

Committee Chair

Tenzing Ingty

Abstract

Biodiversity is fundamental to maintaining ecosystem structure and function globally. Prior to the late 20th century, ecological theory generally predicted that biodiversity would be highest in systems experiencing minimal disturbance. However, Joseph H. Connell challenged this view with the Intermediate Disturbance Hypothesis (IDH), which posits that species diversity peaks at intermediate levels of disturbance and declines under conditions of both low and high disturbance. Although widely cited, the applicability of the IDH to natural systems remains debated, particularly in the context of anthropogenic disturbances and when diversity is measured beyond species richness alone.

This study evaluated the predictions of the IDH in a system influenced by both natural (riverine) and anthropogenic (road) disturbances in central Alabama. Fieldwork was conducted at the Emerald Mountain Expressway bridge spanning the Tallapoosa River in Wetumpka, Alabama. The study area consisted of four 4,000 m² quadrants located at each corner of the bridge. Each quadrant was subdivided into 10 m × 10 m plots arranged along two spatial gradients representing disturbance gradients: distance from the shoreline and distance from the roadway. A combination of random and stratified sampling was employed, with 1 m × 1 m quadrats placed five times within each selected plot. Within each quadrat, all non-woody plant species and their percent ground cover were recorded. Specimens that could not be identified in the field were collected and later identified using voucher specimens at the Jacksonville State University Herbarium.

Biodiversity was quantified using species richness, Shannon’s Diversity Index, and Simpson’s Diversity Index to capture patterns of richness, evenness, and dominance, respectively. Differences across disturbance gradients were analyzed using analysis of variance (ANOVA), while non-metric multidimensional scaling (NMDS) and analysis of similarities (ANOSIM) were used to assess variation in community composition.

Results indicate that species richness, evenness, and dominance exhibited peak values at intermediate distances from the shoreline, consistent with IDH predictions for natural disturbance regimes. In contrast, no significant patterns were detected along the road-distance gradient, suggesting that roadway-related anthropogenic disturbance may exceed the threshold at which intermediate disturbance promotes diversity. These findings support the context-dependent nature of the IDH and highlight differing ecological responses to natural and anthropogenic disturbance regimes.